1. Field of the Invention
The present invention relates to a heating device for heating a member to be heated, by electromagnetic induction, and an image forming apparatus.
2. Description of the Related Art
Conventionally, there has been known a heating device for heating a member to be heated (hereinafter referred to as “to-be-heated member”) by electromagnetic induction. For example, in an image forming apparatus, such as a copying machine or a printer, such a heating device heats a metal roller or a metal belt, which is a to-be-heated member, by electromagnetic induction, and fixes a toner image formed on a sheet using the heat of the heated to-be-heated member.
There has also been known an image forming apparatus of this type, which detects a change in the state of a metal belt as the to-be-heated member, such as a damage, as disclosed in Japanese Patent Laid-Open Publication No. 2007-328159.
FIG. 9 schematically shows a fixing device as a heating device for the image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 2007-328159. The fixing device has an inlet upper roller 6 and an exit upper roller 7 arranged on upstream and downstream sides, respectively, in a manner spaced from each other. A fixing belt 2, which is an endless metal belt, is wound and stretched between the inlet and exit upper rollers 6 and 7. A nip pad 8 for applying pressure to a sheet, and a temperature-detecting thermistor 4 are arranged inside the fixing belt 2. An inlet lower roller 10 and an exit lower roller 11 are arranged on the upstream and downstream sides, respectively, in a manner spaced from each other, and an endless pressing belt 9 is wound and stretched between the inlet and exit lower rollers 10 and 11. A nip pad 12 is disposed inside the pressing belt 9. An antenna 3 for detecting magnetic flux is disposed inside the fixing belt 2 at a location opposite to an induction heating coil 1 with the fixing belt 2 positioned therebetween. The antenna 3 is connected to an energization inhibition circuit, not shown.
The magnetic flux entering the antenna 3 varies depending on the state of the fixing belt 2, and therefore, the image forming apparatus is configured to detect the state of the fixing belt 2 according to the magnetic flux entering the antenna 3 and make the energization inhibition circuit operable to stop the operation of the induction heating coil 1 if there is abnormality in the state of the fixing belt 2.
However, in the image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 2007-328159, the nip pad 8 and the temperature-detecting thermistor 4 are arranged inside the fixing belt 2, and hence there are limitations to the shape and material of the antenna 3. Further, the inside of the fixing belt 2 is under a high-temperature environment, and hence it is necessary to use heat-resistant members which are high in cost, for the antenna 3, which reduces the degree of freedom in design.
Further, normally, to prevent members other than the fixing belt 2 which is the to-be-heated member, particularly members that are not desired to be heated, from generating heat by induction heating, the image forming apparatus is provided with magnetic shields. However, the magnetic shields sometimes adversely affect the antenna 3, causing reduced voltage output from the antenna 3, which sometimes makes it difficult to detect a small magnetic flux.